Prior art applications of controlling the energization of a load have included a triac or bilateral triode switch device which can be made to conduct on both alternations of an AC voltage. In such applications, trigger circuits are provided for the triac which must be triggered into conduction. As an example, in U.S. Pat. No. 3,665,219, there is described an AC semiconductor switch for controlling the energization of a load from a relatively high voltage source which is controlled by signal pulses supplied to the semiconductor from a circuit connected to a relatively low voltage source. Such teachings, however, have the disadvantage that a separate DC voltage source must be provided to the signal producing circuit. This additional DC voltage source tends to degrade the power control due to the cost, reliability and weight associated therewith.
In addition to the above-described disadvantages of the prior art, circuits have been developed to control the energization of a load using techniques such as optical isolation of the trigger signal and the concept of zerocross triggering to improve radiated noise and reduce surge current, for an example, as the circuit described in U.S. Pat. No. 3,723,769. Here again, external DC sources have been utilized.